1. Field of the Invention
This invention relates to a control system either for a power transmission mechanism having a transmission torque capacity varied according to the pressure applied, such as a continuously variable transmission or a friction applying device, or for a power line including the power transmission mechanism.
This invention relates to the subject matter contained in Japanese Patent Application No. 2003-86613, filed on Mar. 26, 2003, No. 2003-114919, filed on Apr. 18, 2003, No. 2003-132224, filed on May 9, and No. 2003-334260, filed on Sep. 25, 2003, which are expressly incorporated herein by reference.
2. Related Art
A belt type continuously variable transmission or a traction type continuously variable transmission transmits a torque by making use of a frictional force between a belt and pulleys or a shearing force of traction oil between discs and rollers. On the other hand, the friction applying device such as clutches or brakes transmits a torque by making use of a frictional force to occur on the surfaces of friction members. Therefore, these power transmission mechanisms are set to have transmission torque capacities according to the pressure to act on the portions in which the torque transmissions occur.
The aforementioned pressure in the continuously variable transmission is called the “clamping pressure”, and the pressure in the friction applying device may also be called the “applying pressure”. When this clamping pressure or applying pressure is raised, the transmission torque capacity can be increased to avoid a slip. On the contrary, there arises a disadvantage that a power is consumed more than necessary for establishing a high pressure or that the transmission efficiency of the power is degraded. Therefore, the clamping pressure or applying pressure is generally set as low as possible within a range of no unintentional slip.
In a vehicle carrying the continuously variable transmission, for example, the mileage can be improved by controlling the speed of an engine with the continuously variable transmission. In order not to deteriorate this advantage, therefore, the clamping pressure is controlled to a level as low as possible within a range of no slip so that the power transmission efficiency in the continuously variable transmission may be improved as high as possible. For this control, it is necessary to detect the pressure to start the slip (i.e., the slip limit pressure). In the related art, various methods are used to detect the slip or the slip limit pressure.
One example is enumerated in JP-A-2001-12593. This discloses a slip detecting method for either the continuously variable transmission for transmitting the power by frictional contacts or its transmission system. In this method, the slip is decided by detecting a rise in the frictional efficiency, as accompanying the lowering of a contacting force (i.e., the clamping pressure or the applying force). The method disclosed in JP-A-2001-12593 is a control method for the continuously variable transmission, which is provided with a pair of conical discs and a driving member to run on the conical discs with wrapping those discs. In this method, the contacting force is gradually lowered while the force to be transmitted, the speed or the transmission ratio being substantially constant, and is stepwise increased when the slip is detected in terms of a rise in an oil temperature. After this, the contacting force is set at a level higher than that at the slip time.
In the invention disclosed in JP-A-2001-12593, moreover, there are stored the characteristic fields, which relate to various different speeds, torques, gear ratios and temperatures and which indicate the contacting force necessary for a specific slip, so that the contacting force between the conical discs is adjusted to correspond to those characteristic fields.
In JP-T-9-500707, on the other hand, there is disclosed a method for detecting a slip in a clutch arranged between an engine and a belt type continuously variable transmission. In this method, a clutch is lowered stepwise from a first pressure level to a second pressure level, and a slight speed difference such as about 50 revolutions is detected so that the slip is decided on the basis of the detection result. In the materials distributed in 7th Luk Symposium 11./12., April 2002, there is disclosed a method for detecting a slip of a belt by varying a belt clamping pressure periodically.
In the method disclosed in JP-A-2001-12593, the slip of the case, in which the clamping pressure is lowered, is detected in terms of the rise in the frictional efficiency. However, a time delay inevitably occurs between the instant, at which the slip actually occurred, and the instant at which the rise in the frictional efficiency is detected in terms of the rise in the oil temperature. Even if the contacting force is stepwise increased by the decision of the slip, therefore, the slip may become excessive. In the method disclosed in JP-A-2001-12593, moreover, the contacting force is increased at the instant when the rise of the frictional efficiency is detected. In case the rise in the frictional efficiency was not detected for any cause, the contacting force is further decreased. As a result, the excessive slip may be caused by the increase in the reduction width of the contacting force.
On the other hand, the torque capacity in the continuously variable transmission is set to correspond to the torque to be inputted to the continuously variable transmission so that the clamping pressure or contacting force corresponds to the input torque. Therefore, the control to determine the slip limit by lowering the contacting force (i.e., the clamping pressure), as disclosed in JP-A-2001-12593, is executed in the actual running state having various input torques.
The prerequisite condition for lowering the contacting force is that the force to be transmitted, the transmission ratio and so on be constant. While the vehicle is running, however, it is ordinary that the running conditions such as the vehicle speed or the torque are varying, although slightly. According to the method disclosed in JP-A-2001-12593, therefore, in case the time period from the lowering instant of the contacting force to the instant of detecting the slip or deciding the slip limit is long, the detection precision of the slip limit may be degraded by the change in the meanwhile drive or running state. Alternatively, the detection of the slip may have to be interrupted. If the lowering gradient of the contacting force is large, on the contrary, an excessive slip or a damage such as wear may be caused by an overshoot.
On the other hand, JP-A-2001-12593 has failed to disclose a specific method for reflecting the slip starting pressure on the proper control of the power transmission mechanism.